In general, underground mine/tunnels/roadways are supported, especially in India, by using wooden cogs/props as support. But as wood is becoming scarce, there is need for suitable alternative supports to replace timbers in mines. Keeping in mind the Indian mining condition the support should be of greater longevity, light enough for easy portability and requiring less maintenance. Further, it should have facilities for height variation, sufficient yielding, continues load monitoring and releasing remotely as the coal seam thickness varies widely in Indian mines. Then, the support material must be easily available and should have ease in operation by even an unskilled worker. Finally, the device must be economically viable for Indian mines, which is thriving for its longer life and should provide safety for miners. In this direction several efforts have been made as can be seen from a survey of the hitherto known prior art.
Reference may be made to U.S. Pat. No. 5,967,702 and CA2236062, wherein is described a pit prop having two opposed ends and comprising two bearing plates located at the ends, two telescopically displaceable prop elements, a number of separate resiliently compressible rubber discs arranged adjacent one another and a tubular indicating element. The prop includes a further bearing plate with the discs being located between two bearing plates to permit yielding of the pit prop under a load. The indicating element has markings thereon to indicate the load condition of the prop. The prop includes a locking device for releasable locking the prop elements with respect to one another.
The drawback of the above said U.S. patent is that the rubber discs may be affected by weathering condition and also will have more wear and tear under repeated loading. Also the prop is a non-yielding rigid prop which is meant for fixed preloading only.
Reference may be made to patent no. GB822162, entitled as “Improvements relating to hydraulic pit props” disclosing a hydraulic pit prop provided with a spring-loaded valve of flow cross-section opening in the direction of liquid pressure and operable either by the liquid pressure to relieve excess pressure in the prop or manually though a longitudinally displaceable rod to initially release the pressure when the prop is to be withdrawn, the valve being interconnected, with axial play, by the rod with a spring-loaded main release valve which opens in the direction opposite to the pressure in the prop, and can be manually opened by means of the rod after the valve has first been opened to initially release the pressure. The coupling between the rod and the valve which comprises a piston on the rod guided in a closed chamber in the influence of liquid pressure in a liquid supply conduit adapted to be connected to the valve casing when the prop is to be set, the valve in this case acting as a non-return valve for the liquid being pumped into the prop.
Reference may be made to patent no. GB2368860, entitled as “Hydraulic prop and seal arrangement” disclosing a hydraulic cylinder especially for use as a pit prop or moving cylinder unit underground, has a locking ring arranged on an outer cylinder tube sealing it at its end with a lead through for an axially movable rod element. The locking ring is provided in the lead through with a sealing arrangement resting on the outer side of the rod element. The sealing arrangement has a guide ring provided with grease grooves open to the outer side of the rod element which can be supplied with lubricant via a lubricant line accessible on the outside of the hydraulic cylinder. The gap formed between an outer collar and the rod element is than the between the rod element and an inner collar.
Reference may be made to patent no. GB1037686 entitled as “Hydraulic operated mine prop, wherein a hydraulic pit prop is disclosed in which at least the external prop member for guiding the sealing portion of the prop piston consists of a longitudinally welded tube. In so far as the sealing member is so selected to accommodate misalignment or relative canting of the prop member then the latter may be made from cheap welded tube stock as opposed to being made from accurately worked, seamless drawn or rolled precision tubes as heretofore. The prop is provided with a retraction spring and the seal with a tensioning sleeve.
Reference may be made to patent no. US2002031407, titled: Prop device for mining and tunnel construction which consists of an inner pipe and an out pipe which can be moved while inserted in each other. The pulling apart motion is initiated when a liquid is filled into a filler opening into the outer pipe whose cavity is separated from the cavity by a plate. This plate features a weak point. Endangerment of the workers is therefore eliminated. After the extension of the inner pipe from the outer pipe the partial wedges are pushed into remaining narrow gap so that the desired locking effect takes immediately place. The water or liquid can exit again from the cavity via the filler opening. A very light prop device that absorbs sufficient straining forces has been created.
Reference may be made to patent no. GB864032, entitled as “Improvements relating to hydraulic pit props or the like” wherein the hydraulic pit prop comprises two telescoping parts which are extended by a differential piston operated by rotating a lever through 180 degrees, and are released by rotating the lever through the remaining 180 degree to open, by means of a projection on the piston, a bolt valve and subsequently a valve member. The prop is first extended by a low-pressure piston and, when its relief valve operates, is set by a high pressure piston. A pressure relief valve is provided.
Reference may be made to patent no. GB742701 as entitled as “Improvements in or relating to telescopic pit-props” wherein the telescoping parts of a pit-prop are locked by independent wedges in slots in a prop lock with or without the interposition of a locking element. The keys are of light alloy are engaged by a galvanized surface on the wedges. The inner prop member may be of double H section having brake shoes adapted to be gripped between the flanges of the inner member by operation of wedges action on both sets of flanges by means of keys. In a modification, each wedge and one key operates on one set of flanges only. For instantaneous withdrawal of the prop an eccentric may be provided or two wedges connected by a sleeve passing over the tapered ends. Reference may be made to patent no. GB948255 entitled as “Improvements in or relating to hydraulic pit props”, wherein the hydraulic pit prop as disclosed is provided with a sealed outer guard tube defining with the cylinder an annular gap which communicates by means of a port with annulus between the piston and cylinder unit the arrangement being such that contraction of the prop is not adversely affected by pneumatic pressure variations in the annulus. The prop is provided with a chamfered base plate bolted thereto. The guard tube may be of corrugated form.
Reference may be made to patent no. GB947516 entitled as “Improvements in or relating to pit props”, wherein the hydraulic pit prop incorporates an auxiliary piston pressurized by an accumulator which supplies pressure for retraction purposes and is charged during extension of said prop. The piston is fixed to the base of the prop and slides in a cylinder, which is fixed to the member therefore and communicates with the accumulator through parts. The accumulator is made of spring steel, plastic or rubber and is mounted in a steel housing. The member is elevated by a pump which feeds liquid from the reservoir through a conduit and valves to a chamber, the underside of the being initially fed with liquid through a valve and then fed with liquid through a valve in the piston. A release valve permits escape of liquid from the chambers to the reservoir when the prop is to be released.
Reference may be made to patent no. GB946631 entitled as “Improvements in or relating to mine-roof supports”, wherein the telescopic pit prop consisting of parts of angular cross-section are interlocked by a tilting ring or clamps comprising pivoted components urged apart by a connected to a double-acting hydraulic jack by arms.
Reference may be made to patent no. GB942471 entitled as “Improvements in or relating to hydraulic roof supports for mines”, wherein the hydraulic pit prop is made up from a number of units each consisting of a rubber ring bonded to steel plates, the units being bolted together and to a control ring provided with inlet, exhaust and relief valves. One plat of each end unit comprises a flanged base or roof support element. In a modification, the prop consists of alternate layers of rubber and steel ring bonded together, one ring acting as a control ring and each end ring as a roof or base plate.
Reference may be made to U.S. Pat. No. 3,992,888 entitled as “Hydraulic pit prop”, wherein the pit prop comprises an elongated skid adapted to lie on the floor and having a front end link able to a support such as a face conveyor. A pair of generally parallel elongated feet flank the skid and each have a front portion, which is substantially twice as wide as its rear portion. A slide fluid cylinder has a rear end secured of the skid and a front end secured to a cross member attached to the front portions of the feet so that extension or contraction of the slide cylinder displaces the feet relative to the support. A pair of support arms each has a rear end loosely pivoted on the rear portion of the respective foot and a front end carrying a roof-engaging shield. A loft cylinder extends between each foot front portion and the respective front end of the respective arm for pressing the shield against the mine roof and the feet against the mine floor.
Reference may be made to U.S. Pat. No. 402,889 entitled as “Pit prop assembly”, wherein the pit prop assembly has two pit props with parallel laterally spaced bases each having a front end turned towards a face conveyor and a rear end turned away from the face conveyor. A slide has its front end connected via positioning mechanism to the conveyor and its rear end connected via hydraulic ram to the bases. Each of the bases has at its rear end a side turned toward a confronting side on the slide and forming a side pair with this confronting side. One of these sides is formed with a formation that engages over and under another formation on the other side. One of these formations of each of the pairs of formation extends longitudinally toward the face so that the two formations can slide relative to each other. One of these formations may also be a pin whereas the other is a groove, or one may be a pair of facing convex surfaces flanking a straight edge.
Reference may be made to patent no. 2454 DEL 95 disclosing a device of support ‘Pit Prop’, which has been developed, and already being used by the mines. It is made up of two pair of steel tubes and a crown. The bottom tubular portion needs a piece of wooden log to be inserted to adjust with the height of mine roofs. A different size of log/piece of timber is required every time to suit the roof height and at the same time a wooden wedge is required at the bottom to tighten the prop rigidly on the ground against the roof. As the wedge is tightened by manual hammering it is not possible to provide a setting load of more then 200-500 Kg. Further, one needs shaping of wooden pieces of different length and thickness to adjust with the roof height from the crown of the prop. It is always difficult and time taking for making a suitable wooden logs for each prop at time of erection, which are hardly used without rework afterwards. Thus, wooden pieces/timbers are paramount for this type of support, which can not be eliminated. Again, wooden wedges, after getting it tightened are responsible for inclination from vertically, which creates eccentricity problem with roof. In addition, yielding characteristic is not available in this pit prop.
Reference may be made to patent nos. GB 968810(A), GB 994903 (A), GB 846291 (A), & GB 699913 (A), entitled as ‘Friction prop’ which is also being used by the mines. The device described herein can provide setting load, height variation and yielding characteristic. The yielding characteristic of friction prop depends upon the frictional forces developed between the outer surface of the inner tube and the inner surface of the vice arrangement provided on the outer tube. Again this frictional force developed depends upon the how perfectly wedges are tightened in the vice arrangement. Further, these friction props are very heavy to handle as they are in a single unit. At the same time remote withdrawal is not possible. If frictional part of the outer surface of the inner tube got damaged it can never be replaced and hence whole prop would be of no use. At the last it does not have load-monitoring device to monitor the upcoming load of roof.
Reference may be made to U.S. Pat. No. 5,967,702 entitled as “Quick-release pit prop”. The pit prop has two opposed ends and comprises two bearing plates located at the ends, two telescopically displaceable prop elements, a number of separate resiliently compressible rubber discs arranged adjacent each other and a tubular indicating element. The prop includes a further bearing plate with the disc being located between two bearing plates to permit yielding of the pit prop under a load. The indicating element has marking thereon to indicate the load condition of the prop. The prop includes a locking device for releasable locking the prop elements with respect to each other.
Reference may also be made to Indian patent No. 1026 DEL 2004 in which the yielding mechanism is a compression helical spring inserted in top tube resting between fixed bottom base and a circular plate on top of the helical spring. Circular plate supports the hollow square canopy through which roof load is transmitted.
In the above-referred props some of the drawbacks are:    a) Longevity of the props is less as the yielding element rubber may be affected by weathering condition as also faster wear and tear.    b) No facilities for giving proper setting load which is a vital parameter to be considered.    c) Roof support area is a point roof support, instead of broad area.    d) Simplest quick release system from a safe distance is not provided.    e) Lack of facility for easy dismantling and good portability.    f) No device to monitor the yielding as well as roof load    g) No facility for replacement of yielding mechanism    h) No facilities for varying the yielding range as in the case of helical spring amount of yielding is fixed.
From the survey of hitherto known prior art it is seen that there is scope for improvement and hence there exists a definite need for providing a device useful for supporting underground mine tunnels/roadways.